Metal sulfide composite materials for batteries

ABSTRACT

Lithium-ion batteries are provided that variously comprise anode and cathode electrodes, an electrolyte, a separator, and, in some designs, a protective layer. In some designs, at least one of the electrodes may comprise a composite of (i) Li2S and (ii) conductive carbon that is embedded in the core of the composite. In some designs, the protective layer may be disposed on at least one of the electrodes via electrolyte decomposition. Various methods of fabrication for lithium-ion battery electrodes and particles are also provided.

CLAIM OF PRIORITY UNDER 35 U.S.C. § 119

The present application for patent claims the benefit of U.S.Provisional Application No. 61/942,285, entitled “Harnessing StericSeparation of Freshly Nucleated Lithium Sulfide Nanoparticles forBottom-up Assembly of High-Capacity, High-Rate Cathodes forLithium-Sulfur and Lithium-Ion Batteries,” filed Feb. 20, 2014, which isexpressly incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States Government support undergrant number W911NF-12-1-0259 awarded by the United States Army. TheUnited States Government has certain rights in this invention.

BACKGROUND 1. Field

The present disclosure relates generally to energy storage devices, andmore particularly to battery technology with active metal ionsparticipating in electrochemical reactions and the like.

2. Background

Owing in part to their relatively high energy densities, light weight,and potential for long lifetimes, rechargeable batteries with activemetal ions are used extensively in consumer electronics, electricvehicles, aerospace and other important applications. The most populartype of rechargeable batteries with active metal ions is lithium (Li)ion (Li-ion) batteries. These batteries have essentially replacednickel-cadmium and nickel-metal-hydride batteries in many applications.Despite their increasing commercial prevalence, further development ofthese and related batteries is needed, particularly for applications inlow- or zero-emission, hybrid-electrical or fully-electrical vehicles,energy-efficient cargo ships and locomotives, aerospace, and powergrids. Such high-power applications will require electrodes with higherspecific capacities than those used in currently existing Li-ionbatteries.

Sulfur (S) and sulfur-containing compounds have been investigated as apotential source for higher specific capacity electrodes, in addition tooffering a number of other advantages, including a high theoreticalspecific capacity (1672 mAh/g of S), high energy density, low voltageoperation, and relative material abundance. Sulfur's specific capacityis the highest among solid cathode compounds known for rechargeableLi-ion batteries and an order of magnitude greater than currentlyavailable commercial cathodes. If used in combination with Li metalanodes, the ultra-high specific capacity of S can enable exceptionalgravimetric and volumetric energy densities in rechargeable batteries(e.g., 2600 Wh/kg and 2800 Wh/l, respectively), which is around 4-10times higher than that of current state of the art Li-ion batteries.Sulfur is also found abundantly in nature, low cost, and light weight,in addition to having a relatively low toxicity.

For all of these reasons, sulfur-based electrodes are being investigatedas a cost-effective, environmentally friendly, performance enhancingcomponent of various batteries, such as Li and Li-ion batteries.Batteries with other active metal ions participating in electrochemicalreactions (e.g., with Na ions, K ions, Mg ions, and other metal ions)may also benefit from the use of high-capacity, sulfur-comprisingelectrodes.

However, realization of the full potential of sulfur-based cathodes inmetal-ion batteries has been hindered by a number of significantchallenges, including low electrical conductivity, low ionicconductivity, and the physical instability of conventional sulfur-basedcathodes. Sulfur and sulfur-containing compounds are highly electricallyinsulating. The ionic conductivity of lithium in sulfur andsulfur-compounds is also very small, which typically slows down theoverall rate of the electrochemical reactions and leads to low powercharacteristics in Li/S cells. In addition, sulfur cathodes generateintermediate electrochemical reaction products (polysulfides, such asLi₂S_(n)) that are highly soluble in conventional organic electrolytes.This leads to sulfur cathode dissolution and re-deposition ofelectrically-insulating precipitates on the anode surface, preventingfull reversibility of the electrochemical reaction.

Thus, despite the theoretical advantages of sulfur-based cathodes,practical application in metal-ion batteries is difficult to achieve.Several approaches have been developed to overcome these difficulties,but none have been fully successful in overcoming all of them. Forexample, some conventional designs have attempted to address the lowelectrical conductivity by using a conductive carbon additive to mix orball mill with S to form C—S composites, but this does not address theionic conductivity or cathode instability. In addition, the uniformityof such composites is poor, which negatively affects their performancecharacteristics in batteries. Similarly, some approaches have mixed orball-milled metal sulfides (e.g., Li₂S) with conductive carbon additivesto form Li₂S—C composites, but these approaches similarly suffer from alack of uniformity and degradation in conventional cells.

Other conventional designs have attempted to address the ionicconductivity by using certain special electrolytes that cause the sulfurto swell, but this often increases the rate of sulfur dissolution. Stillother conventional designs have attempted to melt-infiltrate S into aporous carbon. This approach, however, still suffers from low volumetriccapacity of the produced composites and still has an unsatisfactorilyhigh cathode dissolution rate. In addition, if sulfur expands duringreaction with metal ions (e.g., during reaction with Li ions) formingmetal sulfides (e.g. Li₂S), such an expansion may induce mechanicaldamage within the electrode and electrode particles.

Accordingly, conventional approaches to address sulfur-based (ormetal-sulfide based) cathode shortcomings have found only limitedsuccess. There remains a need for better ways to address the lowelectrical and ionic conductivity as well as physical instability ofsulfur-based (or metal-sulfide based) cathodes in metal-ion batteries.There also remains a need for developing new methods for the formationof uniform metal sulfide composites for use in batteries.

SUMMARY

Embodiments disclosed herein address the above stated needs by providingimproved battery components, improved batteries made therefrom, andmethods of making and using the same.

As an example, a lithium-ion battery is provided that comprises anodeand cathode electrodes, an electrolyte, and a separator. At least one ofthe electrodes comprises a composite of (i) Li₂S and (ii) conductivecarbon. The carbon is embedded in the core of the composite. Theelectrolyte ionically couples the anode and the cathode. The separatorelectrically separates the anode and the cathode.

As another example, another lithium-ion battery is provided thatcomprises anode and cathode electrodes, an electrolyte, a separator, anda protective layer. At least one of the electrodes exhibits a capacityin the range from 50 to 2000 mAh per gram of active material whenoperating within a potential range from about 1 V to about 4 V vs.Li/Li+. The separator electrically separates the anode and the cathode.The electrolyte ionically couples the anode and the cathode. Theprotective layer is disposed on at least one of the electrodes viaelectrolyte decomposition. The protective layer either slows downfurther electrolyte decomposition or reduces reactions between theactive material and an electrolyte solvent.

As another example, a method of fabrication for a lithium-ion batteryelectrode is provided. The method may comprise, for example, forming asacrificial porous thermoset polymer film; impregnating active materialinto the polymer film; thermally treating the active materialimpregnated polymer film in an inert environment to induce carbonizationof the active material impregnated polymer film; and attaching thecarbonized active material impregnated polymer film to a metal foilcurrent collector to form an electrode.

As another example, another method of fabrication for lithium-ionbattery composite electrode particles is provided. The method maycomprise, for example, forming sacrificial porous thermoset polymerparticles; impregnating active material into the polymer particles; andthermally treating the active material impregnated polymer particles inan inert environment to induce carbonization of the active materialimpregnated polymer particles.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are presented to aid in the description ofembodiments of the invention and are provided solely for illustration ofthe embodiments and not limitation thereof.

FIG. 1 is a schematic illustrating a synthesis process according to oneexample embodiment.

FIG. 2 illustrates X-ray diffraction (XRD) results on example materialssynthesized according to the process of FIG. 1.

FIGS. 3A-3F illustrate selected results of electron microscopycharacterization of the produced samples.

FIGS. 4A-4E illustrate example nitrogen sorption curves collected at 77Kon various samples shown for illustration purposes.

FIGS. 5A-5B illustrate example charge-discharge profiles of porous Li2Sproduced according to the above-described synthesis and that ofcommercial Li2S powder collected at relatively high current densitiescorresponding to the C/5 and C/2 rates.

FIGS. 6A-6D illustrate electrochemical performance of example samplesproduced in accordance with the synthesis described above in twodifferent electrolytes comprising different concentrations of an LiTFSIsalt in a DME:DIOX mixture.

FIGS. 7A-7D illustrate the changes in charge-discharge profiles ofLi2S/Li and Li2S-CNT/Li (20 wt. % CNT) cells with cycling in 5M (FIGS.7A-7B) and 7M (FIGS. 7C-7D) electrolytes.

FIGS. 8A-8F illustrate other examples of Li2S and Li2S-graphenecomposite samples produced by precipitation.

FIG. 9 is a graphical flow diagram illustrating an example of a suitablefabrication method for the formation of a carbon shell coated composite.

FIGS. 10A-10D illustrate selected characterizations of example materialsproduced according to the process of FIG. 9.

FIGS. 11A-11D illustrate additional characterizations of examplematerials produced according to the process of FIG. 9 using TransmissionElectron Microscopy (TEM).

FIGS. 12A-12D illustrate electrochemical characterizations of examplecarbon-coated Li2S-graphene composite materials produced according tothe process of FIG. 9 in Li half cells.

FIG. 13 is a graphical flow diagram illustrating an example process forthe formation of C—Li₂S powder via an example ultrafast and scalablebottom-up synthesis route of the type described above.

FIGS. 14A-14B illustrate results of electron microscopy characterizationof example composite samples produced according to the process of FIG.13.

FIG. 15 illustrates the cycling performance of an example compositeC—Li2S electrode produced according to the process of FIG. 13 andcompared with other relevant samples.

FIGS. 16A-16C provide an example illustration of hierarchical particleshelling for enhanced mechanical stability of volume-changing activematerials, such as Li2S.

FIG. 17 is a graphical flow diagram illustrating an example process forthe formation of composite particles with a first level of hierarchy.

FIGS. 18A-18F illustrate select characterizations of example materialsproduced according to the process described in FIG. 17.

FIGS. 19A-19E illustrate additional characterizations of examplematerials produced according to the process described in FIG. 17.

FIGS. 20A-20D illustrates results of select electrochemical tests ofexample materials produced according to the process described in FIG. 17against Li metal foil.

FIG. 21 illustrates results of electrochemical cycle stability tests ofexample materials produced according to the process described in FIG. 17against Li metal foil.

FIG. 22 is a flow diagram illustrating an example method of fabricatinga conductive carbon based electrode.

FIG. 23 is an SEM micrograph illustrating an example Li₂S—C electrodeproduced according to the method described in FIG. 22.

FIGS. 24A-24B illustrate select electrochemical performance results ofthe material of FIG. 23 in half cells with a Li foil counter electrode.

FIG. 25 is a flow diagram illustrating an example method of fabricatingcarbon containing composite particles.

FIGS. 26A-26D illustrate results of select electrochemical tests with aLiI electrolyte additive for in-situ protective shell formation.

FIGS. 27A-27B illustrate results of additional electrochemical testsconducted on a Li₂S—C composite cathode in an LiTFSI/DME:DIOXelectrolyte without and with a LiI additive, as described above andcharacterized in FIGS. 26A-26D.

FIGS. 28A-28D and 29A-29D illustrate post-mortem studies of theelectrodes from the cells of FIG. 27.

FIGS. 30A-30B, 31A-31B, 32A-32B, 33A-33B, and 34 illustrate variousaspects of an example of the favorable performance of the S-basedelectrode in half cells (with a Li foil anode) with LiFSI/DME andLiFSI/DME:DIOX electrolytes.

FIG. 35 illustrates an example metal-ion (e.g., Li-ion) battery in whichthe components, materials, methods, and other techniques describedherein, or combinations thereof, may be applied according to variousembodiments.

DETAILED DESCRIPTION

Aspects of the present invention are disclosed in the followingdescription and related drawings directed to specific embodiments of theinvention. The term “embodiments of the invention” does not require thatall embodiments of the invention include the discussed feature,advantage, process, or mode of operation, and alternate embodiments maybe devised without departing from the scope of the invention.Additionally, well-known elements of the invention may not be describedin detail or may be omitted so as not to obscure other, more relevantdetails.

In the description below, several examples are provided in the contextof Li and Li-ion batteries with Li₂S active materials because of thecurrent prevalence and popularity of this technology. However, it willbe appreciated that such examples are provided merely to aid in theunderstanding and illustration of the underlying techniques, and thatthese techniques may be similarly applied to other types of batterieswith active metal ions other than Li (for example, Na or Na-ionbatteries, Mg or Mg-ion batteries, etc.) as well as corresponding metalsulfides (such as Na₂S, MgS, etc.). Similarly, although several examplesmay be provided in the context of S and Li₂S cathodes, and, in manycases, with certain specific electrolytes, it will again be appreciatedthat such examples are provided merely to aid in the understanding andillustration of the underlying techniques, and that these techniques maybe similarly applied to various other batteries, various electrodematerials, and various electrolytes.

As discussed in the background above, there remains a need in the artfor better addressing the low electrical and ionic conductivity as wellas physical instability of sulfur-based cathodes in metal and metal-ionbatteries. In the view of the inventors, fully-lithiated Li₂S, forexample (having theoretical gravimetric and volumetric capacities of1166 mAh/g and 1935 mAh/cc), is likely to be a significantly morepromising Li storage (active) material because it already contains Liand because it is formed in the already expanded state. The latter mayimprove its stability and handling. However, the preparation of uniformcomposites with Li₂S (for example, preparation of uniform C—Li₂Scomposites) is challenging, particularly if small (e.g., from about 2 to500 nm) and uniform size of Li₂S is desired.

The present disclosure accordingly provides or otherwise facilitates thefabrication and use of improved composite materials comprising sulfur,Li₂S, or other metal sulfides for metal or metal-ion (such as Li or Liion) battery electrodes, improved metal or metal-ion batteries madetherefrom, and methods of making and using such components and devices.In this way, a more full realization of the positive attributes ofsulfur (sulfide) electrochemistry in metal or metal-ion batteries,improved development of advanced sulfur (or sulfide) comprisingcathodes, and improved development of advanced metal or metal-ionbatteries may be achieved. It will be appreciated, however, that someaspects of the present invention may also be applied for electrodes andbatteries that do not contain sulfur, Li₂S, or other metal sulfides, orcontain a mixture of chalcogenides (such as S, Li₂S, or other metalsulfides, other chalcogenides, etc.) with other types of active (ionstoring) materials.

The high melting point of commercial Li₂S (approximately 950° C.) makesit difficult to infiltrate into conductive carbon hosts. For example,ball milling may be used to reduce the size of commercial Li₂S powdersand distribute smaller particles together with carbon additives toimprove their rate performance and capacity utilization. However, theball milling procedure does not allow formation of uniform particleswith controlled morphology or strong bonding between Li₂S and carbon. Asa result of volume changes within Li₂S, the produced cells showundesirably fast degradation. As another example, in order to betterdisperse Li₂S in a carbon host and improve the strength of theirinteractions, lithium polysulfides may be used to synthesize the Li₂S byreleasing H₂S, and the electrodes maintained for 20 cycles. However, H₂Sis a rather toxic gas and harmful to the environment.

By contrast, in one example embodiment of the present invention, Li₂Snanoparticles, porous Li₂S, as well as composites containing both Li₂Sand functional additives, may be produced by the dissolution of Li₂S ina suitable solvent and eventual extraction of the solvent by, forexample, evaporation. An intermediate step of adding a nonsolvent intothe solution and inducing particle precipitation prior to the solventevaporation may also be employed. By the addition of functionalparticles (additives) into the Li₂S solution (e.g., in a suspensionform), Li₂S can be produced incorporating these particles within itsstructure. Depending on the geometry and relative fraction of suchparticles, as well as the chemistry of a particular solution, Li₂Sparticles can also be produced on the surface of these functionaladditives or having functional additives connecting several Li₂Sparticles. In some designs, some of the additives may also completely orpartially encase the Li₂S particles.

The use of functional additives within Li₂S-compising composites mayserve several purposes. For example, the functional additives may beused to enhance mechanical stability (e.g., by minimizing volumechanges) during Li insertion/extraction (during battery cycling), toenhance electrical properties of Li₂S, or to reduce dissolution of Li₂Sduring Li extraction. It is preferable that these functional additivesexhibit small volume changes (for example, less than around 30 vol. %,more preferably below 10 vol. %) during Li extraction from theLi₂S-comprising electrode. When conductivity enhancement is desired,various conductive carbon materials can be used as suitable additivesdue to carbon's low weight, relatively low cost, high conductivity, andhigh Li-ion permeability. In some designs, the surface of the carbonadditives may be coated with various functional groups or moieties. Someof these moieties or functional groups may be introduced to enhancenucleation of polar Li₂S on their surface. In some designs, surfactantsmay also be added into the solution/suspension in order to control thesize and morphology of the produced composites.

Examples of suitable carbon additives include, but are not limited to,carbon black particles, various porous carbon particles (includingvarious types of porous carbons produced using sacrificial templates andself-templating approaches, activated carbon, various porous carbonsproduced from organic and inorganic precursors, etc.), carbon fibers andnanofibers, carbon nanotubes (including multi-walled carbon nanotubes,MWCNT), graphite and graphite flakes, graphene oxide and graphene(including multi-walled graphene and multi-walled graphene oxide),fullerenes, carbon onions, carbon aerogels, dendritic carbons, variousother types of carbon particles and their mixtures, with specificsurface area ranging from around 3 m²/g to around 3000 m²/g. In additionto conductive carbon, conductive metal and conductive ceramic particlesmay also be used. In addition, a carbon precursor may also be used (forexample, a polymer or another suitable organic material) instead ofcarbon particles. Such a precursor may be later transformed toconductive carbon during heat-treatment of the Li₂S-precursor compositeat temperatures below the melting or decomposition of Li₂S (e.g., below950° C.). In the last case, it may be preferable for the carbonprecursor to possess a high carbonization yield, such as a carbonizationyield above 20%. In some cases, it may also be advantageous to usepolymers conductive in the potential range of Li₂S operation within thecell. In order to improve the dispersion of additives, it may beadvantageous to use ultra-sonication or to use mixing.

In some cases, it may be advantageous (either instead of or in additionto conductive carbon) to use active particles (i.e., particles capableof storing or releasing Li ions) as functional additives. Suitableexamples of such particles include various metal oxides, metal sulfides,metal phosphates, and various other known ceramic particles capable ofintercalating Li and exhibiting capacity for Li extraction or insertionabove 10 mAh/g and below 700 mAh/g in the potential range of electrodeoperation within a Li or Li-ion cell. Such particles may alreadycomprise Li in their structure in order to maximize Li capacity of thecomposites. In this case, several advantages may be attained. Most ofthese ceramic particles create stronger bonding with Li₂S and thus mayproduce composites with better mechanical properties. Such particles mayalso capture polysulfides and slow down (or even prevent) Li₂Sdissolution into the electrolyte during Li extraction. If such particlesencase Li₂S, they may be particularly efficient in preventing Li₂Sdissolution into the electrolyte, by allowing Li transport andpreventing a direct contact of the solvent with Li₂S.

It may be important or even critical that the solvent (used for theabove-described preparation of Li₂S nanoparticles, porous Li₂S, as wellas composites containing both Li₂S and functional additives) can bereversibly extracted from the Li₂S solution with the formation of pureLi₂S. It may also be important that the solubility of Li₂S in such asolvent (or solvent mixture) is sufficiently high. In order to reducethe cost associated with solvent use and the energy associated withsolvent evaporation, it may be preferable to use a solvent (or a solventmixture) that has Li₂S solubility in excess of 0.05M, or even morepreferably, in excess of 0.1M. It may also be preferable for the solventto exhibit low viscosity (e.g., below 500 mPa·s, preferably below 100mPa·s, or even more preferably below 10 mPa·s). The inventors haveidentified that ethanol and methanol, for example, work well as solventssuitable for application in the present invention. They offer high Li₂Ssolubility, and do not induce alcoholysis. Further, the viscosity ofethanol is only around 1 mPa·s at room temperature. Therefore, it ispreferred for the Li₂S solution to contain at least 5% (preferably atleast 20%) of their ethanol or methanol or their mixture. Pure ethanolor methanol or their mixture could be used favorably.

FIG. 1 is a schematic illustrating a synthesis process according to oneexample embodiment. In this example, carbon nanotubes (CNTs) are used asfunctional conductive additives. In addition or as an alternative, othercarbon additives may be used as desired, instead of CNTs, as describedabove.

In illustrated schematic, commercial Li₂S is dissolved in ethanol in anultra-dry environment (e.g., within a glovebox with the moisture contentbelow about 1 ppm, preferably below about 0.2 ppm) to obtain anapproximately 0.5 M Li₂S solution in ethanol. Evaporation of ethanolallows for the production of nanoporous Li₂S by consolidation ofindividual Li₂S nanoparticles nucleated heterogeneously from thesupersaturated solution. The addition of CNT into the solution prior toevaporation may induce nucleation and growth of Li₂S heterogeneously onthe CNT surface sites. As a result, the CNT electrically links orconnects multiple Li₂S particles. No gas evolution is generallyobserved, suggesting that H₂S is not released. In order to improve theuniformity of the CNT suspension, an ultrasonic bath treatment may beused. As a further preparation step, all samples may be annealed atabout 100-950° C. (preferably at 200-800° C.) under inert gas or invacuum to remove traces of solvents, and, if needed, to inducedensification. If it is desired to avoid agglomeration of the particles,a lower annealing temperature (e.g., below 800° C., or in some casesbelow 600° C.) may be preferred. In one particular example, it has beenfound that a 400° C. annealing temperature under Ar works well.

FIG. 2 illustrates X-ray diffraction (XRD) results on example materialssynthesized according to the process of FIG. 1. The XRD results confirmthe lack of substantial impurities in the samples (see Li₂S-PDF#26-1188for Li₂S peaks characterization). All of the produced samples in thisexample show the same peaks as commercial Li₂S, indicating the lack ofalcoholysis of Li₂S in an anhydrous ethanol. This is in direct contrastto the known hydrolysis of Li₂S in water, which results in the formationof stable and highly undesirable LiOH.

FIGS. 3A-3F illustrate selected results of electron microscopycharacterization of the produced samples. In particular, FIG. 3A is aScanning Electron Microscope (SEM) micrograph of commercial Li₂S. FIG.3B is an SEM micrograph of nanoporous (nanostructured) Li₂S. FIG. 3C isan SEM micrograph of MWCNT-linked Li₂S with 10 wt. % CNT. FIG. 3D is anSEM micrograph of MWCNT-linked Li₂S with 20 wt. % CNT. FIG. 3E is aTransmission Electron Microscope (TEM) micrograph of MWCNT-linkednano-Li₂S with 10 wt. % CNT. FIG. 3F is a TEM micrograph of MWCNT-linkednano-Li₂S with 20 wt. % CNT.

As shown, the commercial Li₂S displays a particle size of 10-20 μm. Thenanostructured Li₂S powder produced according to the above-describedsynthesis shows smaller porous particles (typically below 5 μm) with50-200 nm interconnected pores present. These pores are produced duringsintering of individual nanoparticles at around 400° C. The crystallinegrains visible within the porous Li₂S often exhibited a cuboid shape andan average size of approximately 200 nm. The SEM micrographs ofCNT-containing samples show most of the Li₂S particles being directlyconnected with CNTs. Defects and non-uniformities within CNTs serve asnucleation sites for heterogenous precipitation of Li₂S. In thisexample, the diameter of the majority of the precipitated Li₂S particlesremains below 0.5 μm, although the size may be less uniform than that ofthe grains in porous Li₂S and the particles are of a more random shape.

The Brunauer-Emmett-Teller (BET) specific surface area (SSA) of theelectrode particles is an important parameter, determining theirelectrochemical properties.

FIGS. 4A-4E illustrate example nitrogen sorption curves collected at 77Kon various samples shown for illustration purposes. In particular, FIG.4A illustrates a BET SSA of commercial Li₂S. FIG. 4B illustrates a BETSSA of MWCNT. FIG. 4C illustrates a BET SSA of nanostructured Li₂S with0 wt. % CNT. FIG. 4D illustrates a BET SSA of an Li₂S-CNT composite with10 wt. % CNT. FIG. 4E illustrates a BET SSA of a Li₂S-CNT composite with20 wt. % CNT.

In this example, the curves are of type II according to InternationalUnion of Pure and Applied Chemistry (IUPAC) classification, which istypical for samples having either no or a very small fraction ofmicropores (e.g., pores less than about 2 nm) and often composed ofeither macroporous adsorbents or nonporous particles. Analysis of theisotherms demonstrates that commercial Li₂S powder exhibitsapproximately 1.7 m²/g of the BET SSA. By comparison, the nanoporousLi₂S powder shows nearly a 20 times larger BET SSA. This corresponds toat least a 20 times shorter diffusion distance for Li ions, which may bebeneficial for cathode performance at high current densities. TheCNT-containing samples show a BET SSA in the range of about 20-30 m²/g,which is consistent with an average Li₂S particle size of approximately100 nm. The relatively high BET SSA of the initial CNTs (approximately100 m²/g) may provide an additional 10-20 m²/g to the CNT-containingsamples if the CNT outer surface is not blocked by the deposited Li₂S.

FIGS. 5A-5B illustrate example charge-discharge profiles of porous Li₂Sproduced according to the above-described synthesis and that ofcommercial Li₂S powder collected at relatively high current densitiescorresponding to the C/5 and C/2 rates. In particular, FIG. 5Aillustrates profiles of commercial Li₂S and nanostructured Li₂S in a 3Melectrolyte at a C/5 rate. FIG. 5B illustrates profiles of commercialLi₂S and nanostructured Li₂S in a 3M electrolyte at a C/2 rate.

The positive impact of the above-described reduction in averagediffusion distance of Li ions becomes apparent when comparing thedischarge profiles of these two samples. The polarization of the firstplateau (from approximately 2.3 to 2 V vs. Li/Li⁺ and corresponding toS→Li_(x)S transition to high order polysilfides) is similar in bothsamples, demonstrating relatively fast Li transport, believed to be dueto pores remaining in the active material (S) after Li extraction fromLi₂S. The second plateau at 2 V vs. Li/Li⁺ corresponds to the conversionof the polysulfides to Li₂S₂. In this example,bis(triflouromethanesulfonyl)imide (LiTFSI) in distilled dimethoxyethane(DME):1,3-dioxane (DIOX) (1:1, v:v) is used as an electrolyte.

FIGS. 6A-6D illustrate electrochemical performance of example samplesproduced in accordance with the synthesis described above in twodifferent electrolytes comprising different concentrations of an LiTFSIsalt in a DME:DIOX mixture. In particular, FIGS. 6A-6B illustrate acomparison of the rate performance of commercial Li₂S, nanoporous Li₂S,and MWCNT-linked Li₂S. FIGS. 6A-6B illustrate a comparison of thecycling performance of commercial Li₂S, nanoporous Li₂S, andMWCNT-linked Li₂S in 5M and 7M electrolytes at room temperature.

Formation of porous Li₂S as well as increasing CNT content in theelectrodes has been shown to improve the accessible capacity and therate performance of the half cells. Even at a moderate charge-dischargerate of C/5, the specific capacity of the cathodes based on commercialLi₂S is 2-3 times inferior to that of the rest of the samples. Thenanosize Li₂S particles linked together with 20 wt. % CNT shows thehighest discharge capacities of up to approximately 1050 mAh/g (per massof S) at the slowest rate of C/20. Similar (possibly slightly higher)stability of the CNT-containing cathodes is observed compared tonanoporous Li₂S. For example, as shown, the cathode containing 20 wt. %CNTs retains approximately 70% of the initial capacity after 100 cycles(vs. approximately 60% for porous Li₂S cathodes) in a 5M electrolyte andapproximately 90% of the initial capacity after 100 cycles (vs.approximately 85% for porous Li₂S) in a 7M electrolyte.

FIGS. 7A-7D illustrate the changes in charge-discharge profiles ofLi₂S/Li and Li₂S-CNT/Li (20 wt. % CNT) cells with cycling in 5M (FIGS.7A-7B) and 7M (FIGS. 7C-7D) electrolytes. As shown, there is a reducedpolarization in cells upon CNT incorporated into the electrodestructure. For example, the use of 20 wt. % CNT reduces the voltagehysteresis from about 0.46 V to about 0.23 V in a 5M electrolyte andfrom about 0.51 V to about 0.37 V in a 7M electrolyte.

FIGS. 8A-8F illustrate other examples of Li₂S and Li₂S-graphenecomposite samples produced by precipitation. In particular, FIG. 8Aillustrates an SEM micrograph of pure Li₂S. FIG. 8B illustrates an SEMmicrograph of an Li₂S-graphene composite with 6 wt. % graphene. FIG. 8Cillustrates an SEM micrograph of an Li₂S-graphene composite with 12 wt.% graphene. FIG. 8D illustrates an SEM micrograph of an Li2S-graphenecomposite with 18 wt. % graphene. FIGS. 8E-8F illustrate TEM micrographsshowing small Li₂S nanoparticles of different shape on a graphenesurface. The inset in FIG. 8F shows a Selected Area Electron Diffraction(SAED) image with diffraction peaks corresponding to differentcrystallographic planes of Li₂S nanoparticles.

In this example, graphene powder was first synthesized by reducinggraphene oxide powder. To prepare graphene-Li₂S composites, commercialLi₂S powder was dissolved in anhydrous ethanol to 0.5 M and graphenepowder was added during continuous mixing of the suspension. No gasevolution was observed. To further improve uniformity of the suspension,it was exposed to an ultrasonic bath treatment for 1 hour. A simplesolvent evaporation method was selected in order to extract ethanol andproduce nanostructured Li2S-graphene composites. The nanoparticles ofLi₂S precipitated heterogeneously on the defective graphene surface. Asa further preparation step, the samples were annealed at 400° C. underAr. Pure Li₂S samples were prepared under the same conditions, butwithout graphene addition.

In some designs, it may be advantageous to deposit a substantiallycontinuous coating layer or “shell” on the surface of theabove-described nanoporous Li₂S or carbon-Li₂S composites, or compositescomposed of Li₂S and other functional additives (such as metals,ceramics, or polymers). Such a coating layer may prevent or otherwisereduce the dissolution of Li₂S during Li extraction. In addition, it mayenhance the mechanical stability of the Li₂S-comprising particles. Insome cases, when the coating exhibits a high electrical conductivity,formation of such a coating may enhance the electrical conductivity ofthe Li₂S-comprising particles.

In some designs, it may be advantageous for such a coating layer tocomprise carbon. In some designs, it may be preferable to deposit acarbon layer in such a way that it does not induce significantadditional agglomeration (or linkage) between the Li₂S nanoparticles,nanoporous Li₂S, or Li₂S-comprising composites. This is because breakageof the agglomerates during electrode preparation may induce damageswithin the carbon coating layer. To this end, a carbon coating may beproduced in several steps. First, a polymer coating may be produced onthe surface of Li₂S nanoparticles, nanoporous Li₂S, or Li₂S-comprisingcomposites. Subsequently, a polymer layer may be carbonized to produceporous carbon. Carbon may then be deposited into the carbon pores (andon the porous carbon surface) by using a gas phase carbon depositiontechnique, such as chemical vapor deposition of the carbon fromhydrocarbon precursors. Examples of suitable hydrocarbon precursorsinclude but are not limited to acetylene, propylene, methane, and manyother hydrocarbons.

In some designs, a protective polymer coating may be produced on thesurface of active particles (such as Li₂S particles, nanoporous Li₂S, orLi₂S-comprising composites in the preceding example, or other activeion-storing particles, such as other conversation-type electrodes andintercalation-type electrodes that require protection against directinteraction with liquid electrolyte) using a process of chemical vapordeposition (CVD). In some designs, the polymer CVD precursors maycomprise monomers and initiators. In some designs, the CVD depositedpolymer coatings may be further carbonized in order to induce formationof carbon coatings. The advantages of polymer CVD include a high degreeof coating uniformity, the ability to deposit a surface coating layerfrom monomers that exhibit different properties in solution (e.g.,produce co-polymers from the hydrophilic and hydrophobic polymers) andthe ability to prevent the use of a solvent (which could be expensive,difficult to remove, or which may have unfavorable interactions with theparticles on which the polymer coating is to be deposited), whichincreases flexibility to design improved composite particles.

In some designs, carbon deposition from the vapor phase may notnecessarily induce undesirable agglomeration. Or, even if suchagglomeration does take place, may not necessarily lead to damaging thecoating during de-agglomeration. For example, if Li₂S is deposited onthe surface of some carbon additives or infiltrated inside the pores ofanother material (e.g., a porous carbon material or porous activematerial), the direct deposition of carbon from the gas phase on thesurface of such materials may be sufficient to produce stablecarbon-coated Li₂S-comprising composite particles. If some of the shellsbecome damaged during electrode preparation, Li₂S in the core may bedissolved (e.g., in ethanol) prior to using carbon-coatedLi₂S-comprising composite particles in the battery electrodes.

FIG. 9 is a graphical flow diagram illustrating an example of a suitablefabrication method for the formation of a carbon shell coated composite.In this example, an Li₂S solution 902 in anhydrous ethanol is used tocoat a graphene layer 904. The coated graphene is then subjected toevaporation to produce an Li₂S nanoparticle 906 and resultant grapheneLiz nanocomposite. The graphene Liz nanocomposite may then be coated(e.g., via CVD) to produce carbon-coated Li₂S nanoparticles 908 and aresultant carbon-coated graphene-Li₂S nanocomposite.

FIGS. 10A-10D illustrate selected characterizations of example materialsproduced according to the process of FIG. 9. Specifically, electronmicroscopy is shown for a produced graphene/Li₂S composite nanopowderbefore CVD (FIG. 10A) and after CVD (FIG. 10B), where carbon-coated Li₂Snanopowder deposited on the surface of a graphite flake (multi-walledgraphene) is visible. Also shown is an X-ray diffraction pattern of theproduced carbon-coated graphene/Li₂S composite nanopowder (FIG. 10C),showing the presence of pure Li₂S, and an Energy Dispersive Spectroscopy(EDS) spectrum recorded on the carbon-coated graphene/Li₂S compositenanopowder (FIG. 10D).

FIGS. 11A-11D illustrate additional characterizations of examplematerials produced according to the process of FIG. 9 using TransmissionElectron Microscopy (TEM). In particular, FIG. 11A shows individualcarbon-coated Li₂S particles on the surface of graphene. FIGS. 11B-11Dshow high resolution images of the carbon coating.

FIGS. 12A-12D illustrates electrochemical characterizations of examplecarbon-coated Li₂S-graphene composite materials produced according tothe process of FIG. 9 in Li half cells. In particular, FIG. 12Aillustrates cycling performance of at C/2 rate at room temperature. FIG.12B illustrates discharge capacity at different C-rates at 35° C. FIG.12C illustrates voltage profiles at different C-rates at 35° C. FIG. 12Dillustrates cycling performance at a C/2 rate at 35° C. LiI was used inthese tests as an electrolyte additive. Exceptional stability and rateperformance are visible.

Incorporating Li₂S nanoparticles within a dense, solid and electricallyconductive carbon matrix may offer particularly attractive properties,such as high stability when used in batteries, high mechanicalstability, high density and high conductivity. However, low-costformation of such composites is challenging, particular if the size ofLi₂S is only within 2-100 nm. Example methods of composite formation mayinvolve (i) synthesis of small, stable and preferably uniform Li₂Snanoparticles, and (ii) either dispersing them within asolvent-compatible carbon precursor solution or depositing carbon ontheir surface via hydrocarbon decomposition. However, the low-costsolvents commonly utilized for organic carbon precursors either dissolveor react with Li₂S. Furthermore, uniform dispersion of tiny Li₂Snanoparticles, which have a natural tendency to agglomerate, ischallenging.

In some embodiments, it has been found advantageous to harness theunique combination of (i) the high solubility of both Li₂S and manypolymers in suitable solvents (e.g., in ethanol), (ii) the stericseparation of freshly nucleated Li₂S nanoparticles in the presence of apolymer, (iii) the bottom-up self-assembly of polymer-coatednanoparticles into larger granules, and (iv) the high thermal stabilityof Li₂S to prepare dense C—Li₂S composite nanoparticles. The inventorshave found that in-situ formation of polymer coatings around freshlyprecipitated Li₂S nanoparticles may efficiently prevent theiragglomeration and growth by steric interactions. The inventors havefurther found that annealing of polymer-Li₂S composites at elevatedtemperatures results in the formation of C—Li₂S composites withoutsignificant side reactions, which may otherwise release toxic CS₂, H₂S,and other S-containing gases, and induce undesirable formation ofvarious Li compounds (such as Li₂CO₃ or LiOH). The C—Li₂S nanopowdercathodes produced by the solution-based methods described herein havebeen shown to provide remarkable performance characteristics and stableperformance in half cells with a Li foil counter collector.

FIG. 13 is a graphical flow diagram illustrating an example process forthe formation of C—Li₂S powder via an example ultrafast and scalablebottom-up synthesis route of the type described above. In this example,commercial Li₂S powder (block 1302) and a polymer (block 1304) are firstdissolved in anhydrous ethanol. The polymer should ideally be polar andsoluble in the same solvent (in this case—in ethanol). Ethanol-solublepolyvinylpyrrolidone (PVP) may be selected as a carbon precursor polymerin this example due to its affinity to Li₂S. Next, both Li₂S/ethanol andPVP/ethanol solutions may be combined together (block 1306) to obtain auniformly mixed solution. Extraction of ethanol from the solution andco-precipitation of Li₂S may be achieved in several routes, such as asimple evaporation. During evaporation of ethanol, nanoparticles of Li₂Sprecipitate first because it may be desirable to select a polymer withhigher solubility in the solvent (the solubility of PVP in ethanol, forexample, is higher than that of Li₂S, which makes PVP suitable for thisprocess). However, due to the strong affinity between the polarprecipitates and high polarity >C═O functional groups of PVP, the Li₂Snanoparticles become simultaneously coated by this polymer, whichprevents the undesirable growth of individual Li₂S particles by a stericseparation mechanism (block 1308). At the same time, self-assembly ofPVP-coated Li₂S nanoparticles induces formation of larger PVP-Li₂Scomposite nanoparticles (block 1310). The last step involvescarbonization of PVP (in this particular example, at 700° C. under Ar)and the formation of C—Li₂S composite nanoparticles (block 1312). Ahigher processing temperature may lead to the undesirable growth of Li₂Sgrains. The mass ratio between Li₂S and a polymer (such as PVP) mayvary, depending on the desired ratio of Li₂S to C in the composite.Suitable values range from 20:1 to 1:10. In one example, a 1:1 PVP:Li₂Smass ratio was used.

FIGS. 14A-14B illustrate results of electron microscopy characterizationof example composite samples produced according to the process of FIG.13. In particular, FIG. 14A illustrates an SEM micrograph of theproduced C—Li₂S composite nanopowder, showing composite particles of100-400 nm in diameter. FIG. 14B illustrates a TEM micrograph of theproduced C—Li₂S composite nanopowder, showing smaller 5-20 nm individualLi₂S nanoparticles within an amorphous carbon matrix. The inset in FIG.14A shows an example of the EDS analysis of the sample. The atomicfraction of Li is estimated assuming Li:S=2:1 stoichiometry

In more detail, the SEM micrograph shows a typical shape and size of theproduced nanoparticles with the characteristic dimensions in the rangeof 100-400 nm. The EDS analysis demonstrates that the weight fraction ofC is approximately 27 wt. %. Since Li was not detected by EDS, in theatomic and weight fraction of Li, S and C elements were estimatedassuming an atomic ratio of Li:S to be 2:1, as also determined from XRDstudies. The TEM studies were used to reveal the distribution of Li₂Swithin these composite nanoparticles and show the microstructure ofcarbon produced from the PVP precursor. The size of Li₂S was found to berelatively broad, but with most particles being small, in the range fromabout 5 to about 20 nm. The introduction of a polar carbonizable polymer(such as PVP) into Li₂S solution facilitated keeping the Li₂S particlesize small. Their distribution within the carbon matrix was uniform.

FIG. 15 illustrates the cycling performance of an example compositeC—Li₂S electrode produced according to the process of FIG. 13 andcompared with other relevant samples. For this electrochemicalcharacterization, a C/5 rate was selected in accordance with otherstudies that utilize this rate in cycle stability tests. Capacityutilization in the produced composites at this rate is unprecedented.Excellent capacity retention was also observed in the C—Li₂S compositenanoparticle electrode, which showed a discharge capacity of over 1200mAhg⁻¹ (per mass of S) after 100 cycles. Such performance originatesfrom the good dispersion of Li₂S within the C matrix, good mechanicalstability of electrodes, and suppressed polysulfide dissolution by theC.

Another aspect of the present invention addresses the need to furtherimprove stability of Li₂S (and, in fact, many other conversion-typeelectrodes for various batteries) by using a hierarchical architectureof the protective shells (protective coatings). In this approach, theshell-protected (coated) composite particles comprise an assembly ofsmaller particles, each having their own protective shells. In somecases, it may be advantageous to have multiple levels of hierarchy. Forexample, having the smaller particles also comprised of the assembly ofeven smaller particles, etc. In this approach, if the protective coatingon the smaller particles fails due to the mechanical stressesaccompanying charge-discharge (or, in one example, Li extraction or Liinsertion), then the particles still have additional protective shell(s)to prevent (or reduce, e.g., by at least twice) undesirableinteraction(s) of active material (e.g. Li₂S) with electrolyte. Inaddition, the use of hierarchical particles allows one to minimizestresses on the outer shell by accommodating some of the strain insidethe composite. It may be advantageous to have the space between theindividual smaller particles of the hierarchical composite filled withan ionically conductive material. If such a material is active (capableof reversibly storing Li ions), this may also be advantageous as it willincrease the capacity of the hierarchical composite. If such a materialis electrically conductive, this may also be advantageous since it willimprove electrical conductivity of the hierarchical composite particles.If such a material possesses some elasticity (at least 1% maximumexpansion), this may also be advantageous since it will help to releasesome of the stresses within the core during charge or discharge of thehierarchical particles.

FIGS. 16A-16C provide an example illustration of hierarchical particleshelling for enhanced mechanical stability of volume-changing activematerials, such as Li₂S. Particles exhibiting three levels of enclosinghierarchy are shown, including zero (small and medium size particles)(FIG. 16A), one (FIG. 16B), and two levels (FIG. 16C).

In this example, both primary and secondary particles are shown to benear spherical for simplicity and illustration purposes. The shape ofthe particles (on various levels of hierarchy) may be irregular,flake-shaped, cylindrical, fiber-like, or other shapes. In addition,Li₂S particles are shown for illustration purposes, but other particles(e.g., Si, Sn, and various other conversion-type anodes for Li-ionbatteries; various conversion-type cathodes for Li and Li-ion batteries,such as metal fluorides (e.g., copper fluoride, iron fluoride, etc.),metal bromides, iodides, their mixtures, etc.; and many other types ofparticles used in batteries, where direct interactions betweenelectrolyte and active material is undesirable) may similarly benefitfrom the illustrated hierarchical shell design as it provides improvedstrength and stability.

In more detail, FIGS. 16A-16C show three levels of hierarchy:zero-level, corresponding to “regular” particles 1602 with a coating1604 (FIG. 16A); first-level, corresponding to multiple particles 1606incorporated within a protective matrix 1608 and further coated with anexternal shell 1610 (FIG. 16B); and second-level, corresponding toshell-coated composite particles 1612, in turn, composed of multipleparticles 1614 incorporated within a protective matrix 1616 and furthercoated with an external shell 1618 (FIG. 16C). Again, however, higherlevels of hierarchy are also contemplated.

In some designs, the shape of the particles at different hierarchicallevels may be different. For example, the overall shape of a compositeparticle may be near spherical, while the shape of the primary particlesmay be flake-like. In another example, the overall shape of thecomposite particle may be flake-like, but the elementary particles maybe spherical. In some examples, it may also be beneficial for particleswith a hierarchical protective shell/coating structure to additionallycomprise other functional additives, as described above. Such additivesmay provide enhanced conductivity (e.g., various carbon additives, suchas graphene, carbon blacks, and other conductive carbon additives) orprovide other benefits, such as improved stability (e.g., by having astronger affinity with volume changing particles, such as Li₂S andexhibiting small volume changes themselves).

FIG. 17 is a graphical flow diagram illustrating an example process forthe formation of composite particles with a first level of hierarchy. Inthis example, (nano)composite particles comprising Li₂S within a polymermatrix are first produced using an approach similar to that describedabove with respect to FIG. 13 (blocks 1702-1704). The particles are thencarbonized in order to produce (nano)composite particles comprising Li₂Sembedded within a matrix of conductive and elastic carbon (preferably,with a pore volume of less than about 0.3 cc/g) (block 1706). In orderto minimize the content of inactive material and maximize the volumetriccapacity of the composite, the volume and mass fractions of the carbonshould be small, preferably smaller than about 50%. In this example, theC—Li₂S (nano)composite (nano)particles are further coated with a layerof carbon (C) by using a chemical vapor deposition (CVD) to createadditional protection, improve the particle mechanical properties, andimprove electrical conductivity of the particles (block 1708).

FIGS. 18A-18F illustrate select characterizations of example materialsproduced according to the process described in FIG. 17. In particular,FIGS. 18A-18B illustrate SEM studies of C—Li₂S composite materialsbefore (FIG. 18A) and after (FIG. 18B) carbon CVD shell formation. FIG.18C illustrates an EDS study of the samples with a C CVD shell. FIG. 18Dillustrates XRD showing the presence of pure Li₂S and the lack ofcrystalline impurities. FIGS. 18E-18F illustrate X-ray photoelectronspectroscopy (XPS) studies of the surface of the particles before andafter CVD shell formation, demonstrating complete coverage of thecomposite particle by the shell.

FIGS. 19A-19E illustrate additional characterizations of examplematerials produced according to the process described in FIG. 17. Thecharacterizations include TEM studies revealing the size of the Li₂Snanocrystals, morphology, and thickness of the disordered graphiteshell. In particular, FIG. 19A illustrates a low resolution TEMmicrograph of the core-shell nanocomposite. FIG. 19B illustrates anScanning TEM (STEM) micrograph showing the projected sample density.FIG. 19C illustrates a normalized Electron Energy Loss Spectroscopy(EELS) profile, showing the presence of a carbon shell along the A-Aline in FIG. 19B. FIG. 19D illustrates a high resolution TEM micrographshowing both the graphitic shell and Li₂S nanocrystals within anamorphous carbon matrix. FIG. 19E illustrates a high resolution TEMmicrograph showing the inter-planar spacing within the nanocrystalscoinciding with that of the (111) planes of Li₂S.

FIGS. 20A-20D illustrate results of select electrochemical tests ofexample materials produced according to the process described in FIG. 17against Li metal foil. In particular, FIG. 18A illustrates rate-testperformance at room temperature, 35° C., and 45° C. FIGS. 18B-18Dillustrate the shape of the charge-discharge curves collected atdifferent rates and at different temperatures, showing high rateperformance and a small value of the hysteresis.

FIG. 21 illustrates results of electrochemical cycle stability tests ofexample materials produced according to the process described in FIG. 17against Li metal foil. As shown, the results demonstrate substantiallyhigh capacity utilization and virtually no degradation within 100cycles.

In still other aspects of the present invention, a novel architecture ofthe electrodes, a novel route to produce electrodes with improvedproperties, and a novel route to produce individual composite particlesare provided. Conventional routes to produce electrodes typicallyinvolve formation of active particles (e.g., Li₂S-comprising particles),mixing these particles with a polymer solution and conductive carbonadditives to achieve homogeneous slurry, casting this slurry on thesurface of a metal foil current collector, and drying. There is a needto increase the electrode thickness (and mass loading of the activematerial per unit area) in order to reduce the weight and volumefraction of inactive components of the battery (such as separators andmetal foils) and also to reduce the cost of the coating process.However, when drying thick coatings, they tend to crack and delaminatefrom the current collector. In addition, increasing the coatingthickness undesirably increases electrical resistance of the electrode(in the direction perpendicular to the electrode) due to point contactsbetween the active particles and conductive additives.

FIG. 22 is a flow diagram illustrating an example method of fabricatinga conductive carbon based electrode. In this example, the method 2200includes formation of a film (e.g. a free-standing film) of a porousthermoset polymer or polymer mixture (block 2202); impregnating orinfiltrating this film with active material (or a precursor(s) of activematerial) (block 2204); optionally (if needed or if needed as a separatestep) converting the precursor(s) into active material particles(optional block 2206); and heat-treating the produced composite atelevated temperatures to induce carbonization of the polymer and itsconversion to conductive carbon (block 2208). To reduce the amount ofinactive material within the electrode, it may be preferable that thefinal amount of carbon does not exceed 25 wt. % (even more preferably,15 wt. %). Because the polymer chains of the polymer are rigid, theproduced electrode does not crack during the solvent evaporation (e.g.,if a solvent is used for active material infiltration) even if theoverall electrode thickness is large (e.g., within 60-200 microns).Because the produced carbon electronically connects all particles withinthe electrode, the achieved conductivity and mechanical stability of theproduced electrode may be very high. In order to protect the surface ofthe active material particles from undesirable interactions with theelectrolyte, it may also be desired to optionally deposit a protectivelayer on the surface of the active material (optional block 2210).Carbon deposition (e.g., by CVD) is an example of such a protectivecoating method. Carbon deposition also further increases the electricalconductivity of the electrode.

As a particular example, a carbonizable polymer may be impregnated witha solution of Li₂S in anhydrous ethanol, the produced material may bedried from the solvent, and the process optionally repeated as necessaryto achieve a desired polymer-Li₂S ratio. Subsequently, the polymer-Li₂Scomposite may be carbonized in an inert environment (e.g., under Ar gas)to produce a C—Li₂S composite. The composite may be further coated witha layer of CVD carbon using a hydrocarbon precursor (e.g., acetylene).The polymer-Li₂S composite may also be coated with another layer ofpolymer (e.g., by using CVD) before or after carbonization. Andsimilarly, another layer of CVD carbon may be further deposited.

Here, the produced electrode may be able to protect high capacity Li₂Sparticles against dissolution or unfavorable reactions with theelectrolyte, while achieving high electrode uniformity, high electrodeconductivity, and high electrode rate performance. Furthermore, thismethod allows for the production of uniform electrodes with high massloading (high capacity loading) per unit electrode area.

FIG. 23 is an SEM micrograph illustrating an example Li₂S—C electrodeproduced according to the method described in FIG. 22. As shown, a veryhigh degree of electrode uniformity and smoothness is visible. Remainingpores within the electrode may be used to achieve rapid transport ofelectrolyte ions.

FIGS. 24A-24B illustrate select electrochemical performance results ofthe material of FIG. 23 in half cells with a Li foil counter electrode.The illustrated results include rate performance (FIG. 24A) and cyclestability (FIG. 24B). Remarkable stability and ultra-high capacity isdemonstrated.

In addition to the preparation of the electrode as described above withreference to FIG. 22, a modification of this approach is suitable forthe fabrication of the individual particles.

FIG. 25 is a flow diagram illustrating an example method of fabricatingcarbon containing composite particles. In this example, the method 2500includes producing porous polymer particles (block 2502) andimpregnating (infiltrating) with active material (or a precursor(s) ofactive material) (2504). The next optional step (if needed or if neededas a separate step) includes converting the precursor(s) into activematerial particles (optional block 2506). Further heat-treatment of theproduced composite particles may be conducted at elevated temperatures(e.g., 300-1000° C., sufficiently high to carbonize a polymer but not sohigh that damage to the active material occurs) in order to inducecarbonization of the polymer and its conversion to conductive carbon(block 2508). To reduce the amount of inactive material within thecomposite electrode particle, it may be preferable that the final amountof carbon does not exceed 25 wt. % (even more preferably, 15 wt. %). Inorder to protect the surface of the active material particles fromundesirable interactions with the electrolyte, it may also be desired tooptionally deposit a protective layer on the surface of the producedcomposite (optional block 2510). Carbon deposition (e.g., by CVD,preferably at temperatures below 800° C.) is an example of such aprotective coating method. Polymer deposition (e.g., by CVD, preferablyat temperatures below 350° C.), with or without subsequentcarbonization, is another example of such a protective coating method.Carbon deposition (or deposition of a polymer electrically conductiveduring exposure to electrolyte and during operation within the cell)also further increases the electrical conductivity of the electrode.

In some applications, it may be advantageous to infiltrate differenttypes of active material in a single porous polymer particle. In thiscase, for example, a combination of favorable properties enabled byindividual components of active material may be achieved. For example,one active material may offer higher energy density, while anotheroffers higher capacity or higher rate performance.

In some applications, it may also be advantageous to additionallyinfiltrate another polymer or an organic compound into the pores of thepolymer in addition to the infiltration of the active material. In thiscase, for example, formation may be induced of the (porous) carbon filmon the surface of active particles during heat-treatment.

In the two examples above, Li₂S particles are used for illustrationpurposes, but other particles may also greatly benefit from using thedisclosed electrode fabrication methods. Examples of suitable activematerials may range from Si, Sn, Sb, P, SiOx, and various otherconversion-type anodes for Li-ion batteries, to various metal fluorides(e.g., iron fluorides, copper fluorides, bismuth fluorides, etc),various metal bromides (e.g., lithium bromide), various selenides,sulfides, iodides and various other conversion-type cathodes for Li andLi-ion batteries, to various intercalation-type active materials to beused as anodes or cathodes in Li-ion batteries and electrochemicalcapacitors, to various electrochemically active polymers, to variousother types of materials that can be used to store energy viaelectrochemical reactions.

In yet another aspect of the present invention, unwanted reaction of theactive material with liquid electrolytes may be substantially reduced byforming protective coatings in-situ, during the initial cycles of celloperation. As discussed above, Li₂S cathodes, for example, typicallysuffer from polysulfide dissolution, which reduces the cell capacity andincreases cell resistance by (i) increasing electrolyte viscosity andion mobility and (ii) inducing the continuous precipitation of resistiveLi₂S and Li₂S₂ on the surface of Li in a Li₂S—Li cell. In some of theembodiments described above (e.g., with respect to FIG. 9 and FIG. 13),a Li+ conducting shell may be formed on the Li₂S surface, which mayprotect Li₂S against dissolution during Li extraction, with carbon beingan example of a suitable shell. However, it may desirable for certainapplication to have decreased complexity and a lower cost of cellfabrication. Further, some applications may be less tolerant of defectswithin the protective layers or require increased toughness. Whilehierarchical shells address this concern, they nonetheless furtherincrease complexity of a system and may increase material synthesiscost.

To address the need for an efficient yet low-cost approach to protectthe electrode surface from liquid electrolyte (or, in some cases toprotect the electrolyte from continuous decomposition on the electrodesurface), the formation may be induced of an effective protectivecoating on the surface of cathode particles (e.g., Li₂S-based) in-situ,which helps to keep battery costs low and ensure high coatinguniformity. This approach is complementary to the protective shellformation. In some cases, additional benefits can be achieved withelectrolyte modifications to induce this in-situ film formation.Examples of such benefits include, but are not limited to, thefollowing: (i) the overpotenal of first charge may be reduced and (ii)the cell rate performance may be significantly enhanced. It may bepreferred for this in-situ formed protective layer to reduce the rate ofundesirable reactions (such as continuous electrolyte decomposition,partial etching, or dissolution of the cathode) by at least two times,preferably by at least four times, even more preferably by ten times.

Although the formation of an in-situ protective layer on the lowpotential electrodes (e.g., with a Li insertion potential below 1 V vs.Li/Li+) have been used in conventional designs (e.g., on graphite or onLi foil anodes), the application of an in-situ formed protective layeron the medium-high voltage electrodes (e.g., electrodes with activematerials having a Li insertion potential between 1 V and 4 V vs.Li/Li+, such as Li₂S and many others, and having specific capacities inthe range of about 50 to about 2000 mAh/g when operated within thispotential range) have not been employed and, at most, thought to beunnecessary, useless, or harmful. Furthermore, there have been only afew examples when in-situ (during the cell operation) formed coatingscould be formed at medium-high voltages (e.g., between 1 V and 4 V vs.Li/Li+, where Li₂S, Li₂Se, various metal fluorides, otherconversion-type electrodes, and some intercalation-type electrodesoperate) and it was at best unclear if such coatings would provide anybenefits to cell performance (including the cells with electrodescomprising Li₂S or other conversion-type active materials). Therefore,such embodiments are believed to be novel and important. Finally, thein-situ formation of a superior protective layer, as provided by certainaspects of this description through the use of certain electrolyte saltsor electrolyte salt additives, or through a combination of salts andadditives, is also significant for battery applications. Such superiorprotective layers may benefit various types of electrodes (not onlythose that are active in the medium-high voltages of 1-4 V vs. Li/Li+).

One example of a suitable electrolyte is the DME-based electrolytecomprising LiI as an additive. Some other ethers may also be usedinstead of DME as electrolyte solvents. Similarly, additives other thanLiI may induce reduction of the electrolyte at the potential above thatof the Li insertion into the de-lithiated Li₂S (or othermedium-high-voltage electrodes, 1-4V vs. Li/Li+).

FIGS. 26A-26D illustrate results of select electrochemical tests with aLiI electrolyte additive for in-situ protective shell formation. In thisexample, coin cells were assembled with an Li₂S—C composite cathode andan approximately 4 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)salt solution in distilled dimethoxyethane (DME):1,3-dioxolane (DIOX)(1:1, v:v) with 0, 0.2, 0.5 or 1 M LiI used variously as an additive, asdescribed in relevant detail below.

As shown, the voltage profiles of the two initial charge-dischargecycles of such Li₂S—C electrodes against Li without (FIG. 26A) and with(FIG. 26B) LiI additives (0.5 M in this case) into electrolyte (lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI) salt solution in1,2-dimethoxyethane (DME):1,3-dioxolane (DIOX) solvent mixture) showsignificant differences. Without the additive, the first charge profileof nanostructured C—Li₂S based electrode displays a visibleover-potential in the range of 3.0-3.8V, previously explained by a highactivation barrier for Li extraction from crystalline Li₂S. Addition ofLiI dramatically reduces overpotential. To be clear, because the redoxpotential of LiI is approximately 3V vs. Li/Li+, iodine dissolved in theelectrolyte does not contribute to capacity during charge-dischargecycling. The positive impact of LiI on the reduction of first cycleoverpotential (as seen from a comparison of FIG. 26A and FIG. 26B) maybe explained by three separate hypotheses: (i) LiI acts as a redoxmediator, with the primary function to enhance electrical conductivityof the electrode in contact with electrolyte by transferring electronsfrom conductive carbon to the Li₂S surface through the ionic mediator(within its oxidation potential within 2.9-3.4 V vs. Li/Li+); (ii) LiIimproves ionic conductivity through the solid electrolyte interphase(SEI) initially formed on the Li anode and (if it exists) on thecathode. The fact that these electrodes show near-theoretical capacity(see FIG. 26A-26B) suggests that they already possess sufficientconductivity and that further conductivity enhancement (e.g., viaintroduction of redox mediators into the electrolyte) is unlikely tolower the first cycle overpotential significantly. Furthermore, becauseI-does not oxidize below approximately 2.8 V vs. Li/Li+ due to the highthermodynamic potential of LiI, LiI may not be very active as a redoxmediator when charging the cell to below 2.8V (first charge). Finally,the hysteresis for the second cycle in both electrodes is nearlyidentical at this rate, suggesting little difference in the overall cellresistances. Since the ionic conductivities of electrolytes slightlyincrease with LiI additions, there remains no evidence of electrodeconductivity enhancement. Therefore, in the view of the inventors, thesimplistic redox mediator theory (i) is unlikely. LiI-based solid stateelectrolytes are known to exhibit high ionic conductivity ranging from10⁻⁷-10⁻³ S/cm and, indeed, improve ionic conductivity and reduce theover-potentials involved in various electrochemical reactions during thefirst charge. However, in this case it may be expected that the amountof LiI added into the electrolyte should have a major impact on thefirst cycle overpotential. To test this hypothesis, a very small amount(0.01 M LiI) was added into the electrolyte, and a much smaller Liextraction potential was still observed (approximately 2.85 V with 0.01M LiI compared to approximately 3.3 V without). Without reliance on anyparticular theory for operation, it is at least believed that either SEIproperties are improved in the presence of LiI for reasons other thanLiI inclusions or electrocatalytic activity of LiI is responsible forthe decrease in the activation energy of Li₂S oxidation and theresulting reduction in the first cycle overpotential. Because iodides(including LiI) have been previously used as catalysts for variousprocesses, ranging from H₂O₂ decomposition to various organic reactions,the last hypothesis is not unreasonable. Cyclic voltammetry (CV) curves(FIG. 26C and FIG. 26D, collected at a scan rate of 0.05 mV/sapproximately equal to C rate around C/10) confirm the results ofcharge-discharge tests and show that the hysteresis between 1st chargeand discharge decreased substantially when the LiI was used as anelectrolyte additive. Without using the LiI additive, the cathodic peakis very broad in the range of 3.1-3.5V, indicating a high energy barrierfor the activation, which is consistent with the high over-potential inFIG. 26A. With the LiI additive, the first cathodic peak shows verysharp peak centered at a noticeably lower potential of 2.9V. The sharperpeak during the second cycle indicates better kinetics of Li ions'diffusion in LiI containing cells, which may be a result of both thecatalytic effect and slightly better SEI properties (FIG. 26D).

FIGS. 27A-27B illustrate results of additional electrochemical testsconducted on a Li₂S—C composite cathode in an LiTFSI/DME:DIOXelectrolyte without and with a LiI additive, as described above andcharacterized in FIGS. 26A-26D. In particular, FIG. 27A illustrates rateperformance for Li₂S-based electrodes in 5 M LiTFSI and 5 M LiTFSI withdifferent molar concentrations of LiI as electrolytes. FIG. 27Billustrates cycling stability performance recorded at C/5 for Li₂S-basedelectrodes in 5 M LiTFSI and 5 M LiTFSI with different molarconcentrations of LiI as electrolytes.

As shown, the reduced first cycle over-potential reduces side reactionsand correlates with the suppressed polysulfides' dissolution, which areboth significantly positive attributes for the Li/S cells. Forcharge-discharge (C-D) tests, the impact of LiI on the rate performancebecomes apparent at around 1C or faster rates. The reversible capacitiesare increased by approximately 100 mAh/g at C/20, C/10, and C/5, and byover approximately 300 mAh/g at 2C due to the LiI additive. Ratecapability of the Li₂S-based electrode with a 0.5 M LiI electrolyte wastested at room temperature from C/20 to SC. Even at a high C rate of 1Cthe cell shows highly reversible capacities of 1114 mAh/g (normalized bythe mass of S). After rate testing, the cell returns to C/2 cycle testwith the capacity coming back to 1233 mAh/g. Both excellent ratecapability and good stability were evident and attributed to the goodreaction kinetics in the electrode based on nanostructured C—Li₂Scathode materials and good stability of this electrochemical system byusing LiI. By using LiI as an additive, capacity retention was increasedfrom 76% to 85% and 96% after 100 cycles. The best performing cell (0.5M LiI) exhibited 1310 mAh/g capacity after 100 cycles at C/5, which isremarkably high considering that no polysulfides were added to theelectrolyte. When compared to the recent literature reports, thecombination of high capacity utilization and cycle stability of the cellcomprising 0.5 M LiI is outstanding.

FIGS. 28A-28D and 29A-29D illustrate post-mortem studies of theelectrodes from the cells of FIG. 27. These studies reveal a dramaticeffect of LiI on the morphologies of both Li and Li₂S electrodes.

FIGS. 28A-28C show the SEM images of fresh Li anode foil (FIG. 28A) aswell cycled (100 cycles at C/5) Li foils without (FIG. 28B) and with(FIG. 28C) a 0.5 M LiI additive. The fresh lithium foil has asubstantially smooth surface. After cycling in a regular electrolyte,the surface becomes rough and covered with flake-like crystalline Li₂Sgrowing like dendrites on it, which is confirmed by X-ray diffraction(XRD) of cycled lithium foil exhibiting very sharp diffraction peaks ofLi₂S (FIG. 28D). The formation of such an SEI as a result of polysuldifeshuttle not only reduces cell capacity by reducing the content of activematerial on the cathode side, but also increases the cell's resistance.In sharp contrast, the use of LiI yielded a very smooth surface of thecycled lithium foil with no dendrites and Li₂S particles visible after100 cycles (FIG. 28C). The SEM study also reveals a net-like porousstructure of the Li SEI layer, which may be beneficial for fasterdiffusion of Li ions. The XRD study does not show any new crystallinephase formed after 100 cycles (FIG. 28D), providing further evidencethat polysulfide shuttle was strongly reduced in cells comprising LiI.This SEI evidently prevented (or at least significantly reduced) thereduction of polysulfides to Li₂S on the surface of lithium foil, whichsimilarly enhances the cycling performance, in addition to prevention(or at least significant reduction) of polysufide dissolution by theprotective film formed on the S cathode discussed below.

FIGS. 29A-29D illustrate SEM analyses of cycled cathodes of cells fromthe electrochemical tests of FIG. 27. A similarly dramatic differencecan be seen in the sample morphology with the LiI addition. The surfaceof the cathode comprising C—Li₂S composite and a small content ofnanosized carbon black additives becomes covered with large (0.5-3 μm)size crystals after cycling in a regular electrolyte. These largecrystals are precipitates of dissolved polysulfides (Li₂S or Li₂S₂),which are shuttling between cathode and anode sides during cycling. Thisprecipitation layer is insulated for the electron and Li ions and canblock the Li ions during the lithiation or delithiation, which couldcontribute to the rise in the cells' resistance and reduced utilizationof the active material. The LiI additive yielded markedly differentcathode morphology (FIG. 29C). After cycling, the cathode surfacebecomes coated with a very smooth polymer-like film, which is insignificant contrast to the rough surface of the fresh electrode(compare FIG. 29C and FIG. 29A). No crystalline precipitates areobserved, further suggesting that polysulfide diffusion through such aprotective film layer was significantly impeded. Not unexpectedly, bothenergy dispersive spectroscopy (EDS) and XPS tests revealed the presenceof I on the surface of this cycled cathode and the distribution of I wasvery uniform. The irreversible mass loss of S in the cathode electrodesafter 100 cycles reflects the degree of the polysulfide dissolution. Thechange in the S/S0 weight ratio in the cathodes after cycling, asdetermined from the EDS measurements, show as much as approximately 20%losses of S in the electrode after 100 cycles in a baseline electrolyte.In contrast, adding LiI into the electrolyte reduced the S losses byseven times to only approximately 3% after 100 cycles. Combinations ofexperimental evidence reveal that the addition of LiI to the electrolyteinduces formation of a protective film on the surface of bothelectrodes.

In addition to LiI, other metal halide (e.g., LiF, LiCl, MgF₂, MgI₂,etc.) additives may also be used in some electrolytes for the in-situformation of an efficient protective layer for Li-ion and otherbatteries.

Another example of a suitable electrolyte is the DME-based electrolytebased on LiFSI salt. This salt is not particularly stable and maydecompose with the formation of LiF and radicals. Such radicals mayinduce polymerization of organic electrolytes and produce a protectivesurface layer.

Other salts (e.g., MgFSI, LaFSI, or LaTFSI salts; other salts containingrare earth metals; Li salts not containing FSI-anions; or salts of othermetals that do not contain FSI-anions) that either produce metal halidesor radicals or both upon in-situ (within a cell) decomposition may beutilized for certain embodiments (e.g., for the formation of protectivecoatings on the electrode surface or improving the properties of suchprotective coatings). Such a decomposition may be triggered byincreasing temperature, by reduction, or by oxidation of the suitablesalt. This concept of purposely using “unstable” salts that decomposein-situ is markedly different from the state of the art because one ofthe parameters often used for electrolyte salt selection is itsstability in a broad potential range (the range electrolyte is exposedto in a cell). Taking advantage of the “unstable” salts that decompose,produce radicals (or produce metal halides or both) and initiateformation of the suitable protective layers on the electrode surface(particularly on the surface of the medium high voltage electrodes) istherefore believed to be novel. To be clear, this concept should not beconfused with the concept of using solvent additives that may induce afavorable solid electrolyte interphase on the anode surface. However, itis noted that such concepts may be combined in certain embodiments ofthe present invention, where an improved surface layer may be achievedby combining electrolyte solvents (as main electrolyte solvents or asadditives) that may induce cross-linking with the salts that produceradicals or metal halides upon decomposition (as additives or as mainsalts) in a single electrolyte.

FIGS. 30A-30B, 31A-31B, 32A-32B, 33A-33B, and 34 illustrate variousaspects of an example of the favorable performance of the S-basedelectrode in half cells (with a Li foil anode) with LiFSI/DME andLiFSI/DME:DIOX electrolytes. Similar to the previously described exampleof LiI additive-comprising electrolyte, this example demonstrates thatelectrochemical reduction of a lithium bis(fluorosulfonyl)imide(LiFSI)-based electrolyte and interaction between polysulfides andFSI-anions allow for the formation of a very effective protectivecoating. This coating suppresses the S shuttle (with columbicefficiency, CE, being nearly 100%) and allows S—Li cells to exhibit verystable long-term cycling performance (1000 cycles+) during acceleratedtests conducted at 60° C. Neither electrolyte additives norprior-to-testing electrode coating technology were utilized for the cellpreparation.

FIGS. 30A-30B show results of selected electrochemical testing. One ofthe most popular solvents in Li—S batteries is 1:1 vol. ratio of1,3-dioxolane (DIOX) and 1,2-dimethoxyethane (DME) binary solvent with 1M LiTFSI salt. A cyclic DIOX is used due to its ability to improve apassivation layer on a Li metal anode, while DME with its higher donornumber (DN=20) compensates for the low solvation ability of DIOX. LiTFSIsalt is used in nearly all research on Li—S because this salt iscommonly believed to resist unfavorable interactions with polysulfides,while the use of other more conductive salts (such as lithiumbis(fluorosulfonyl)imide (LiFSI), LiPF₆, LiBF₄, etc.) degrades cellperformance. For example, a recent study of various ionic liquid(IL)-based electrolytes for Li—S cells showed that the unfavorablereactions between polysulfides and both [FSI]- and [BF₄]- anions resultin a dramatically faster fading of cells comprising such anions in ILelectrolyte. In contrast to this conventional understanding, theinventors have discovered and demonstrated that an LiFSI salt dissolvedin organic solvents may offer an outstanding performance for Li—S andrelated chemistries by inducing a protective surface coating layer. Inaddition to their higher conductivity, LiFSI-based electrolytes offerlower solubility of polysulfides. FIGS. 30A-30B illustrate results ofthe accelerated cycle stability tests conducted at 60° C., which revealsa major impact of substituting a common LiTFSI salt by LiFSI. Asexpected, cycle stability tests of Li—S cells in 5 M LiTFSI solution inboth pure DME and DME:DIOX mixtures reveal relatively rapid degradation;after 200 cycles these cells lost approximately 70% and 60% of theirinitial capacities, respectively. Slightly better performance of theDME:DIOX mixture may be explained by the DIOX ability to form a betterpassivation layer on a Li foil electrode, which reduces the irreversibleLi₂S/Li₂S₂ losses on its surface. Contrary to rapid fading ofLiTFSI-based cells, significantly better capacity retention was observedin LiFSI-based electrolytes. In this case, the addition of DIOX as aco-solvent results in a surprisingly higher first cycle charge capacitycompared to the performance of pure DME, which the inventors hypothesizeto be related to the reduced polysulfide dissolution in DIOX-containingcells. However, in spite of the higher initial capacity, theDIOX-containing cells show a relatively fast degradation. In an effortto determine a possible degradation mechanism, the inventorsinvestigated possible changes in electrolytes during their storage insealed glass containers. It was found that DIOX-containing LiFSIelectrolytes (LiFSI/DIOX and LiFSI/DME: DIOX) showed a gelation behaviorin less than a day even at room temperature. Since both LiTFSI-basedelectrolytes and LiFSI/DME did not exhibit this behavior, it is proposedthat a bulk cationic polymerization of DIOX by more polar LiFSI thanLiTFSI is a probable gelation cause. Higher viscosity of LiFSI/DME: DIOXshould indeed reduce the polysulfide dissolution, but the growth inviscosity during electrolyte polymerization over time will lead to anincrease in the ohmic resistance and may lead to an eventual failure(here, the cell failed to operate at the 175th cycle). In order to avoidpolymerization of the bulk electrolyte, a cell with LiFSI in pure DMEwas prepared. This cell retained 86% of the initial capacity after 200cycles. This is significant considering the high operating temperature.Further cycling up to 1000 cycles showed that capacity fading sloweddown over time and total capacity loss was only about 35% after 800cycles. Interestingly, when the LiFSI concentration was reduced from 5 Mto 3 M in order to increase polysulfide solubility in the electrolyte,it was observed that in spite of the lower initial capacity (asexpected) the cycle stability improved with a capacity loss of about 24%after 1000 cycles. Higher Li salt concentration (5 M LiTFSI) result inhigher specific capacity (due to the more suppressed polysulfidedissolution), but less stable performance (due to the formation of aless favorable SEI on Li, which exhibits higher and slowly growingresistivity). After the cell performance is stabilized at around the100th cycle, the cell with 3 M electrolyte showed only 0.07 mAh/g ofaverage capacity drop per cycle. More importantly, approximately 100.0%average CE was observed in both 3 M and 5 M LiFSI/DME cells, confirmingthat polysulfide dissolution and the resulting “S shuttle” waseffectively suppressed.

FIGS. 31A-31B illustrate cyclic voltammetry conducted on cells preparedusing bare aluminum (Al) foils as working electrodes in Li half cells. Asignificant reduction is seen of LiFSI-based electrolytes at highvoltages (a first peak at approximately 2.2V and a second peak atapproximately 1.6V vs. Li/Li+) at 60° C. However, at room temperaturethe same LiFSI-based electrolytes (including LiFSI/DME) did not show anypeaks, indicating that at these electrolyte salt concentrations highthermal energy is required to initiate the observed high voltageelectrolyte reduction process. Furthermore, conventionally usedLiTFSI-based electrolytes in different solvents did not show highvoltage electrolyte decomposition at both 25° C. and 60° C., suggestingthat electrolyte reduction is triggered by LiFSI. FIG. 31B shows thereduction is self-limiting.

FIGS. 32A-32B illustrate scanning electron microscopy (SEM) micrographsof the S-impregnated carbon cathode and Li anode surfaces after cyclingfor 150 times, which shows a dramatic morphological difference betweenthe LiTFSI and LiFSI-based electrolytes. The initial carbon morphologywith sharp particle edges and a rough layer of deposited (poly)sulfideson the surface is visible for the cells with a traditional LiTF SI-basedelectrolyte, consistent with previous observations of typical S—Ccathodes after cycling. In sharp contrast, a smooth layer of protectivefilm containing spherical shaped-decomposed products covered the S—Ccathodes cycled with the LiFSI electrolyte, supporting a high degree ofdecomposition by LiFSI and electrochemical observations. The Li surfacein cells cycled with LiFSI electrolyte was covered uniformly with thedecomposed electrolyte layer and showed dramatically reduced dendriteformation. In contrast, the morphology of the Li anodes in cells cycledwith LiTFSI electrolyte was irregular and dendritic, demonstratinguneven Li plating/deplating.

FIGS. 33A-33B illustrate the positive impact of the LiFSI salt on Licycling stability. Tests were conducted of repetitive plating/deplatingof Li with 5 M LiTFSI and LiFSI in DME single solvent using symmetricLi—Li cells. At both room temperature and 60° C., cells withLiTFSI-based electrolyte showed higher activation overpotential andlarge spikes in voltage, possibly due to the lack of SEI former (DIOX).In contrast, LiFSI-based electrolyte furnished stable cyclingperformance and lower overpotential. This result indicates the abilityof LiFSI/DME to efficiently passivate the Li metal.

FIG. 34 illustrates the impact of the film formation on room temperaturecycling of the S—Li cells in 5 M LiFSI/DME electrolyte. At roomtemperature, a faster capacity decay can be seen than at 60° C., eventhough the dissolution rate at room temperature should be lower.However, when the first cycle of the cell is conducted at 60° C. and therest of the cycles at room temperature, stable cycle performance wasobserved with high specific capacity. These results correlate well withthe high temperature electrolyte reduction process observed at 60° C.,suggesting that a protective film is only formed at elevatedtemperatures in the disclosed LiFSI-based electrolytes and is needed tostabilize the cell at room temperature. Based on the collected evidence,it can be concluded that the formation of FSI(-F) anion radicals atelevated temperatures (60° C.) and the subsequent radical-inducedreactions with polysulfides and polymerization of DME is largelyresponsible for the in-situ formation of the efficient long-termprotection layer on the surface of S cathodes. This protective surfacelayer is evidently permeable to Li ions, while being impermeable topolysulfides resulting in approximately 100% CE.

Accordingly, the use of LiFSI electrolytes in Li and Li-ion cells(particularly comprising low potential, less than 4 V vs. Li/Li+)electrodes may induce formation of the surface layer (on the lowpotential electrode(s)), which may stabilize either the electrode(s) orthe electrolyte, or both against unfavorable surface reactions. Whensuch a surface layer formation is induced, the cycle stability andoverall performance of the cells may be significantly enhanced. Solventsother than DME may also be used in combination with LiFSI for thein-situ formation of the protective layer.

While commercial cells utilize 0.8-1 M salt concentration inelectrolytes, the inventors have shown that high electrolyteconcentration (molarity above 2 M) may provide additional benefits, suchas improved cell stability either due to reduced reactivity betweenelectrodes and electrolyte solvents or due to favorable interactionsbetween the concentrated salts and solvent molecules, which may lead tothe formation of the protective surface layers in-situ. Such highmolarity electrolytes are particularly useful for the compositeparticles described in certain embodiments herein or in electrolytecompositions described in other embodiments herein. High electrolyteconcentration may also lead to stable use of metal anodes (such as Lianodes), particularly when the electrolyte comprises LiFSI salts, LiIadditives, or other metal halide additives, such as LiF, LiBr, MgBr,MgF₂, and others. It will be appreciated though, that such additivesshould not induce corrosion of the counter electrodes or the currentcollectors and should not induce significant undesirable redox shuttlefor a given cell chemistry.

FIG. 35 illustrates an example metal-ion (e.g., Li-ion) battery in whichthe components, materials, methods, and other techniques describedherein, or combinations thereof, may be applied according to variousembodiments. A cylindrical battery is shown here for illustrationpurposes, but other types of arrangements, including prismatic or pouch(laminate-type) batteries, may also be used as desired. The examplebattery 3500 includes a negative anode 3502, a positive cathode 3503, aseparator 3504 interposed between the anode 3502 and the cathode 3503,an electrolyte (not shown) impregnating the separator 3504, a batterycase 3505, and a sealing member 3506 sealing the battery case 3505.

The invention claimed is:
 1. A lithium-ion battery, comprising: anodeand cathode electrodes, wherein at least one of the electrodes comprisesa core-shell composite of (i) a Li₂S active material and (ii) functionaladditives, wherein the functional additives comprise a metal oxide and aconductive carbon, with the metal oxide and the carbon being embedded inthe core of the composite, and wherein the metal oxide is an activematerial that exhibits a capacity above 10 mAh/g in an operationalpotential window of the at least one electrode; an electrolyte ionicallycoupling the anode and the cathode; and a separator electricallyseparating the anode and the cathode.
 2. The battery electrodecomposition of claim 1, wherein the composite further comprises aprotective shell arranged to impede polysulfide dissolution during Liextraction from the Li₂S.
 3. The battery electrode composition of claim2, wherein the protective shell comprises electrically conductivecarbon.
 4. The battery electrode composition of claim 2, wherein atleast a portion of the protective shell comprises a material depositedby chemical vapor deposition.
 5. The battery electrode composition ofclaim 1, wherein the Li₂S comprises a precipitate from a solution driedat temperatures in excess of about 100° C.
 6. The battery electrodecomposition of claim 5, wherein a solvent of the solution includes atleast 20 wt. % ethanol or at least 20 wt. % methanol.
 7. The batteryelectrode composition of claim 5, wherein the solution comprisessuspended additive particles.
 8. The battery electrode composition ofclaim 7, wherein the additive particles are conductive carbon particles.9. The battery electrode composition of claim 5, wherein the solutioncomprises a sacrificial polymer provided for conversion to conductivecarbon upon annealing at temperatures in the range of about 400° C. andabout 800° C.
 10. The battery electrode composition of claim 9, whereinthe solubility of the polymer in the solution is higher than that ofLi₂S, and wherein the polymer is polar with a substantially highaffinity to Li₂S.
 11. A lithium-ion battery, comprising: anode andcathode electrodes, wherein at least one of the electrodes exhibits acapacity in the range from 50 to 2000 mAh per gram of active materialwhen operating within a potential range from about 1 V to about 4 V vs.Li/Li+; a separator electrically separating the anode and the cathode;an electrolyte ionically coupling the anode and the cathode; and aprotective layer disposed on at least one of the electrodes viaelectrolyte decomposition and comprising electrolyte decompositionproducts, whereby the protective layer either slows down furtherelectrolyte decomposition or reduces reactions between the activematerial and an electrolyte solvent.
 12. The battery composition ofclaim 11, wherein the electrolyte comprises a lithium halide.
 13. Thebattery composition of claim 12, wherein the lithium halide is lithiumiodide.
 14. The battery composition of claim 12, wherein lithium halideis lithium bromide or lithium fluoride.
 15. The battery composition ofclaim 11, wherein the electrolyte comprises a metal salt that producesradicals during in-situ decomposition.
 16. The battery composition ofclaim 11, wherein the electrolyte comprises a metal salt that producesmetal halides during in-situ decomposition.
 17. The battery compositionof claim 16, wherein the electrolyte comprises lithiumbis(fluorosulfonyl)imide.
 18. A method of fabrication for a lithium-ionbattery electrode, comprising: forming a sacrificial porous thermosetpolymer film; impregnating active material into the polymer film;thermally treating the active material impregnated polymer film in aninert environment to induce carbonization of the active materialimpregnated polymer film; and attaching the carbonized active materialimpregnated polymer film to a metal foil current collector to form anelectrode.
 19. The method of claim 18, wherein the thickness of theelectrode is greater than about 80 microns.
 20. A method of fabricationfor lithium-ion battery composite electrode particles, comprising:forming sacrificial porous thermoset polymer particles; impregnatingactive material into the polymer particles; and thermally treating theactive material impregnated polymer particles in an inert environment toinduce carbonization of the active material impregnated polymerparticles.
 21. The battery electrode composition of claim 1, wherein themetal oxide is an active material exhibiting a capacity in the range ofabout 10 mAh/g to about 700 mAh/g.
 22. The battery electrode compositionof claim 21, wherein the metal oxide is an active material exhibiting acapacity in the range of about 100 mAh/g to about 250 mAh/g.
 23. Thebattery electrode composition of claim 1, wherein the electrolytecomprises a non-aqueous electrolyte.